A diving bell is often used as a habitat for divers working at extreme depths for extended periods. The bell is internally pressurized to the pressure of the surrounding water, so that divers can enter and leave the bell without undesirable pressure changes. When working in cold water it is desirable to heat the bell for diver comfort, or the effective working time is greatly reduced. Electrical heating requires considerable power, which may be impractical to supply from the surface, and for which batteries are inadequate. To maintain heat in the bell it is necessary to recirculate some of the breathing gas, rather than rely on a constantly replenished supply from the surface. When recirculation is used, excess carbon dioxide must be removed from the breathing gas. Scrubbers for this purpose are available, but require forced flow to be effective and so necessitate another source of power.